Adding a driver after initial rental car pick-up
 

I am renting a car at JFK but am picking up an "additional driver" the day after I get it somewhere else.

Assuming I go to one of the company's locations with the individual in question so that they verify their driver's license, this should work, right?

Does anyone have any experience doing this? Thank you.

Yes, generally all you have to do is go to a location with the additional driver physically present.